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The guano islands off the coast of Peru are home to some of the world’s largest seabird colonies. Millions of cormorants, boobies, and pelicans come here to take advantage of the fish-filled Humboldt Current, making for an awesome spectacle. Equally awesome, if smellier, are the mountains of guano they leave behind.

As fertilizer, guano has been a significant international commodity for well over a century; buoyed by a robust export market in the mid-1800s, Peruvians enjoyed decades of guano-based peace and prosperity. But the Guano Era, as it is sometimes called, began drawing to a close in 1864, when the Spanish invaded the islands as part of an effort to reassert control over their former colony. War with Chile over guano and nitrate deposits followed in 1879. It was not until 1909, when the Peruvian government nationalized the islands—including Los Checos, seen here—that relative peace descended, ending decades of overexploitation.

Guano is still sold as an alternative to synthetic fertilizers, and owing to strict conservation mea- sures, the harvest has been largely sustainable. Those measures are increasingly being flouted, however, and the guano birds face other pressures as well.

Climate variability has stressed them badly, and their populations have declined from more than 50 million in the 1800s to about 4 million today. The local anchoveta fishery—the largest in the world—has also competed directly with the birds.

“Peru is the OPEC of fish meal,” says Patricia Majluf, a biologist who has worked to protect the islands since 1979. The Peruvians responded in 2009 by creating the Guano Islands and Capes National Reserve, establishing much-needed limits on the anchoveta take. “The fishing industry is happy because the lower catch keeps prices high,” says Majluf. “And the birds are happy because they have fish to eat. So we will have guano for a long time.”